Abstract Plant Growth Regulator Sprays and Girdling: Potential Horticultural Techniques to Increase Fruit Retention and Yield of Longan (Dimocarpus longan Lour.) Trees in California Leila Graves June 2009 The longan is a subtropical fruit tree whose popularity is growing quickly among American consumers, and thus, could become a lucrative niche market for California growers. Trees planted as part of an initial variety trial conducted in commercial California orchards are beginning to bear fruit. However, fruit retention and yields are low due to excessive abscission during early fruit set. Horticultural techniques, such as plant growth regulator (PGR) sprays and girdling, often increase fruit retention and yield of fruit tree crops, though effects are typically dependent on treatment timing and PGR concentration. The objective of this study was to identify PGR and girdling techniques that result in increased longan fruit retention. The experiment was conducted in a commercial orchard using a randomized complete block design with whole-tree experimental units and five replications per treatment. Treatments included foliar applications of two synthetic auxins, 1-naphthaleneacetic acid (NAA) at 20, 60, or 100 ppm or 2,4-dichlorophenoxyacetic acid (2,4-D) at 5, 25, or 45 ppm, each applied during early or late fruit set. Girdling was also conducted at early or late fruit set, for a total of 15 treatments, including an untreated control. Factors examined at harvest included fruit number and mass per inflorescence and individual fruit mass and diameter. During the on-crop year, a significant increase in the number and mass of fruit per inflorescence was observed in response to 25 ppm 2,4-D applied at late fruit set, as compared to the control. Despite this increase in fruit number, 25 ppm 2,4-D applied at late fruit set did not result in decreased diameter and mass of individual fruit. Late fruit set applications of 25 ppm 2,4-D appear to mitigate the effects of alternate bearing in the off-crop year. Fruit abscission during early and/or late fruit set decreased in response to 5 ppm 2,4-D at early fruit set, 25 or 45 ppm 2,4-D at early or late fruit set, or 100 ppm NAA at early fruit set. Fruit loss occurred as a result of Santa Ana winds during both growing seasons. Neither leaf nor fruit nutrient concentrations of treated trees were significantly different from that of untreated controls. Results indicate that properly-timed PGR applications have the potential to be used commercially to increase yield and profit to longan growers in California.